Fluid bed expansion and fluidisation

ABSTRACT

Method and apparatus in which a biomass is carried on a substrate that is fluidised. The fluidised substrate has a layer of distributor particles below it, through which the fluidizing medium passes prior to contact with the fluidised substrate and biomass associated therewith. The effect of the distributor layer is that the particles damp out excessive turbulence in the fluidizing medium, thus preventing undue turbulence of the medium within the fluidised layer. The particles may be recycled through the distributor layer to strip excess biofilm from the substrate particles prior to returning the substrate particles to the fluidised bed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/492,495 filed on Sep. 24, 2004, now U.S. Pat. No. 7,309,433, theentire disclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to improvements in the design and operation ofexpanded or fluidised beds in which a fluid, particularly a liquid, isused to transmit the energy for bed expansion. An expanded or fluidisedbed is one in which the particles are suspended in a fluid flow but donot substantially move with the bulk flow of that fluid. The classicalchemical engineering definition of an expanded bed is one that isincreased in volume up to 50% or 100% over that of the bed when static,i.e., with no fluid flow; whilst a fluidised bed is defined as having avolume more than 50% or 100% greater than that of the static bed with nofluid flow. In particular, the present invention relates to biologicalprocesses such as water and wastewater treatment, fermentation, andbio-catalysis. For such processes, areas in need of improvement includedistribution of the liquid flow; energy costs for pumping and aeration;control of biomass overgrowth; and biomass support materials

Recent publications in the scientific literature have highlightedaspects of fluidised bed design and operation, which are in need ofimprovement. For example, P. M. Sutton and P. N. Mishra (“Activatedcarbon based biological fluidised beds for contaminated water andwastewater treatment: a state of the art review”, Water Science andTechnology Vol. 29 10-11: 309-317, 1994) point out that “The mechanicalcomponents and sub-systems critical to the design of BFB (biologicalfluidised bed) commercial system embodiments are the following” and wenton to cite the distributor, oxygen transfer, and control of biofilmgrowth. Their paper was based on a review of “Over 80 commercial,media-based BFB reactors (that) have been installed in North America andEurope.”

In a more recent review, C. Nicolella, M. C. M. van Loosdrecht and J. J.Heijnen (“Wastewater treatment with particulate biofilm reactors”,Journal of Biotechnology 80: 1-33, 2000) identified four keydisadvantages of fluidised bed operation:

1. Biofilm formation on carriers, which poses problems leading to longstart-up times.

2. Difficulty in control of biofilm thickness.

3. Overgrowth of biofilm leading to elutriation of particles.

4. High cost of liquid distributors for fluidised systems forlarge-scale reactors and associated problems with respect to cloggingand uniform fluidization.

Since the introduction of liquid fluidised bed technology, a number ofpatents have been granted for devices to ensure the uniform distributionof liquid flow at the base of the bed. These include downward flowthrough expansion nozzles (e.g., U.S. Pat. No. 4,202,774, U.S. Pat. No.4,464,262, U.S. Pat. No. 4,618,418, U.S. Pat. No. 5,584,996); nozzleswith a perforated grid or plate above (U.S. Pat. No. 4,702,891, U.S.Pat. No. 4,933,149); perforated distributor plates, similar to thoseused in conventional gas-fluidised systems (U.S. Pat. No. 4,322,296) orwith a static bed of coarse sand and fine grades of sand above (U.S.Pat. No. 5,965,016); or simply a static bed of granular material (U.S.Pat. No. 5,895,576), sand (GB780406) or both (GB2149683).

If the fluid flow at the base of the bed is turbulent, this results inincreased impacts between fluidised particles producing abrasion, or inthe case of particles carrying a reactant layer, premature stripping ofthe reactant layer from the fluidised carrier particles.

GB780406 discloses a particulate distributor comprising a static bed ofsand lying on a perforated screen with flow rates of the order of 1gallon per square foot per minute (49 L per m² per mm) or an upwardvelocity of about 0.08 cm per cm per sec. This low rate of flow throughthe distributor is insufficient to cause movement of its particles, andthe teaching here is that the granular material is being used in effectas a “3-dimensional” perforated plate. In an attempt to improve thefluid flow characteristic in a fluidised bed, Bernard Suchowski, JosephE. Gargas, Robert H. Hyde and Joseph Pluchino (U.S. Pat. No. 5,965,016)proposed the use of larger and heavier particles of sand collected justabove a perforated distributor plate, where they help distribute theflow more evenly.

In spite of this, the presence of the perforated plate itself posesphysical constraints to fluid flow.

BRIEF SUMMARY OF THE DISCLOSURE

We have found that by removing the plate completely and causing orallowing the particles of the distributor layer to move, but notthemselves to be fluidised, a significant improvement in fluid flowproperties in the lower part of the bed results.

In one aspect of the present invention, there is provided a method forimproving the performance of a fluidised bed in which a bed ofparticulate material is fluidised by the passage of a fluidizing mediumthere through, characterised by the provision of a distributor layerthrough which the fluidizing medium is caused or allowed to pass priorto passing through the fluidised bed, the density of the particles ofthe distributor layer and the flow rate of the fluidizing medium beingselected such that turbulence in the fluidizing medium is substantiallyreduced or eliminated before acting on the fluidised bed.

In a particular aspect of the present invention, a reactant moiety ofthe fluidised bed may be carried as a film or layer on a particulatecarrier. In this particular case, the thickness of the reactant layer onthe inert carrier medium may be controlled by allowing particlescontaining excess of reactant on the surface to be removed from theupper part of the fluidised bed to be recycled into the distributorlayer, whereby excess biomass material is stripped from the carrierparticles by the action of the distributor layer as the carrierparticles pass therethrough towards the fluidised bed. Moreover, it hasalso been observed that biofilm thickness control can be achievedwithout recycling particles from one end of the bed to the other end.

Interactions at the interface between the moving bed distributor and thefluidised bed cause stripping of excess biofilm, resulting in a morecompact biofilm. Evidence for the more compact nature of the biofilmarises from observations that the degree of bed expansion reduced (from117.5 to 98.0 cm) but the static bed height remained substantially thesame (52.9-53.3 cm); and that the bed began to compact more rapidly,once settled from the expanded state, during the period over which thiseffect occurred.

The invention provides in a further aspect an apparatus for improvingthe performance of a fluidised bed, which apparatus comprises:

means for establishing a bed of material to be fluidised, and

injection means for injecting a stream of fluidizing medium through saidbed,

characterised by the provision of a distributor layer of particulatematerial through which the fluidizing medium is passed substantiallyprior to passing through said bed whereby turbulence in the fluidizingmedium as it passes through said fluidised bed is substantially reduced.

The distributor layer may be a layer of a particulate material having adensity greater than that of the particles constituting the fluidisedlayer itself. In a particular embodiment of the present invention thedistributor layer is agitated by the fluidizing medium but is not itselffluidised. What is required is that the particles constituting thedistributor layer move with the flow of fluidised medium but the beditself is not fluidised. In this way, the layer of distributor materialacts to constrain the turbulence of the flow of fluidizing medium, whichturbulence is damped by the movement of the particles in the distributorlayer. As a result, the fluidizing medium serves to fluidize the bedwithout undue turbulence or violent movement of the particlesconstituting the bed. In this way, if the bed constitutes a delicatematerial, abrasion or damage to the particles constituting the bed isreduced to a minimum.

The distributor layer in a preferred embodiment of the invention is amoving distributor layer in which the movement of the particles of thedistributor layer serves to distribute more evenly the flow of fluidmedium to the underside of the fluidised bed while at the same timeserving to damp out turbulence within the fluid flow. The result is asubstantially lamina flow of fluid through the fluidised bed whichserves to open the structure of the fluidised bed to permit interactionbetween the fluidizing medium and the particles constituting the bedthus promoting interaction between the two while at the same timereducing to a minimum the severity of collisions between particleswithin the fluidised bed.

This overcomes a long-standing problem of fluidised bed technology,where hitherto the strong movement of fluidizing material within thecentre of the bed or in juxtaposition the fluid inlet has causedexcessive agitation of the bed and the lack of uniformity of reactionconditions across it. As discussed in the introduction to thisspecification, a significant amount of technology in terms of nozzles,injection means, baffle plates and the like have been used in an attemptto overcome this problem.

The distributor layer may be a distinct layer below the fluidised bedand the overlap between the two layers is preferably at a minimum toreduce abrasion and/or removal of the reactant from the carrierparticles, although some minimal interaction is to be encouraged as itserves to control the biofilm and allow its development in a morecompact form. Typically, the fluidizing medium may be a liquid.

It will be appreciated by the person skilled in the art that thethickness of the distributor layer and the fluidizing medium flow ratemay be selected such that substantially no turbulence is experienced inthe fluidised reactant bed as a result of passage of the fluidizingmedium therethrough.

The precise parameters constitute something of a balancing act. Thedenser the particles of the distributor layer, the greater the amount ofenergy necessary to produce appropriate movement within the distributorlayer. Thus, there is a trade-off between density of the particles andsize of particles in the distributor layer with the pressure/velocity ofthe fluidizing medium feed. This balance is also important in gainingthe benefit of biofilm control solely through interactions at theinterface.

The present invention has been found to be particularly useful influidised bed or fermentation reactions involving biological material.In a particular aspect of the present invention the fluidised bedparticles may be coated with a biofilm layer as a reactant moiety.

According to one aspect of the invention, in this particular case theparticulate material constituting the fluidised bed particles may be theinert carrier medium for the biofilm. In a further aspect of theinvention the inert carrier medium may be a glassy coke, upon thesurface of which the biofilm layer is immobilised.

Such a material is described and claimed in our No GB99/03542, thedisclosure of the specification of which is incorporated herein byreference. Cells grow best on slightly porous materials, which enablesthem to adhere and the biofilm to develop, while at the same timeproviding the largest possible surface area. Glassy cokes, produced bythe high-temperature treatment of bituminous coals, tend to give thebest results. What is required is a coke with at least a slightly glassyor vitreous surface. This results in a material that has a surfacesubstantially impervious to the passage of mineral matter from withinthe coke to the biofilm layer thereon. The presence of the “glassy”surface, therefore, serves to protect the biofilm from the effects ofinjurious minerals and compounds frequently present in cokes. Typically,however, the coke will have a substantially uniform composition and theglassy nature of the coke will not be limited to the surface only. Theglassy coke particles may have a size substantially within the range of0.25 to 2.50 mm., in a preferred embodiment the particle size may bewithin the range of 1.0 to 1.7 mm.

In many processes, particularly where a biomass is involved, as, forexample, in a wastewater treatment, fermentation or biocatalyticprocess, the reactant in the fluidised bed is a film of biomass materialcarried as a layer on the particles of a particulate fluidised carrier.This invention has particular application to such arrangements since anextremely valuable feature of the invention is that it allowssubstantially automatic control of the thickness of the reactant layeron the inert carrier medium. In a bioreactor, the layer of biologicalmaterial carried by the individual particles of the fluidised bed isencouraged to grow and to reproduce. As a result, the overall density ofthe particles (including the biomass or biofilm layer) is reduced with aresult that those particles having an increased biofilm thickness on thesurface will tend to be carried upwardly through the bed towards theupper part of the reactor vessel and will eventually be carried out ofthe vessel itself.

We have found that by redirecting this feed from below the exit of thereactor vessel and re-introducing the particulate and biomass materialat the base of the reactor vessel, the effect of the distributor layeris to strip the outer biomass material from the surface of the particlesand allow the particles to resume their place in the lower portion ofthe fluidised bed where the residual biomass will begin, once again, inthe fertile conditions within the bed, to produce a further biofilm.

In an alternative embodiment, the balance between the inlet velocity andhence momentum of silica sand can effect biofilm control substantiallyby interactions between the moving bed distributor material and thefluidised bioparticles.

The recycled particles may be combined with fluidizing medium and priorto introduction to the distributor layer, or in an alternativeembodiment, the recycled particles may be injected separately into thedistributor layer.

In this latter case, it is preferred that the temperature of the liquidis controlled to within the range of 13 to 22 degrees centigrade,typically, 14-21° C.

In a further embodiment of the present invention the fluidised bed maycontain an upper denitrification layer above the nitrification layer.This additional layer incorporates denitrifying bacteria to break downthe nitrites and/or nitrates produced by the aerobic nitrifying bacteriain the lower nitrification layer. By allowing the nitrification toproceed almost to a stage in which the liquid, in this case water, isalmost completely de-oxygenated, the conditions are ideal for anoxicconversion of the resulting nitrite/nitrates to nitrogen gas thuseliminating the nitrite/nitrates from the liquid.

The microbes involved in denitrification are normally bacteria capableof “anaerobic respiration”, that is, bacteria that respire using oxygen,but also have the ability to use chemically combined oxygen whenmolecular oxygen (O₂) is at low concentration or absent. Sources ofchemically combined oxygen that bacteria and archaea can utilise includenitrite (NO₂ ⁻), nitrate (NO₃ ⁻), sulphate (SO₂ ⁻) and carbonate (CO₃²⁻) and, by operating the nitrification reactor in such a way as toremove all the dissolved molecular oxygen, denitrifying microbes canrespire the nitrite and/or nitrate. In this way, and if process controlis adequate, nitrite and/or nitrate is converted to molecular nitrogen(di-nitrogen, N₂), which returns to the atmosphere (air=78% N). Whenprocess control is inadequate, the intermediates in the reduction ofnitrate may be released into the atmosphere. Suitable denitrifyingbacteria may be one or more of:

Achromobacter piechaudii (Alcaligenes piechaudii), Achromobacterruhlandii (Alcaligenes ruhlandii), Achromobacter xylosoxidans subsp.denitrificans, Alcaligenes denitrificans, Alcaligenes xylosoxidans,Azoarcus tolulyticus, Azoarcus toluvorans, Azospirillum brasilense(Spirillum lipoferum) Azozoarcus toluclasticus, Bacillushalodenitrificans, Blastobacter aggregatus, Blastobacter capsulatus,Blastobacter denitrWcans, Candidatus “Brocadia anammoxidans”, Comamonasdenitrificans, Flavobacterium sp. Flexibacter canadensis, Haloferaxdenitrificans (Halobacterium denitrificans), Halomonas campisalis,Hyphomicrobium denitrificans, Jonesia denitrificans, (Listeriadenitrificans) Kingella denitrificans Neisseria denitrificans,Ochrobactrum anthropi, Paracoccus denitrificans (Micrococcusdenitrificans), Pseudoalteramonas denitrificans (Alteromonasdenitrificans), Pseudomonas denitrificans, Pseudomonas putida,Pseudomonas stutzeri, Roseobacter denitrificans, Roseobacter litoralis,Thauera aromatica, Thauera chlorobenzoica, Thiobacillus denitrificans,Thiomicrospira denitrificans, Thiosphaera pantotropha.

This list is not, however, exhaustive.

The relatively deoxygenated water may be exhausted from the top of thefluidised bed and caused or allowed to overflow or cascade as a thinfilm to effect rapid aeration of the medium. Alternatively, it may berecycled through a counter-current aerator, i.e. downwards from or nearthe top of a narrow column; where air, oxygen-enriched air, or pureoxygen is bubbled upwards from at or near the bottom. The columndiameter being sized such that the downward velocity of the liquid to beaerated or oxygenated is slightly less than the natural rise velocity ofthe gas bubbles, which is typically 22 centimeters per second for a 2 mmdiameter air bubble rising in quiescent water and 42 cm⁻¹ for a swarm ofbubbles. In this way, the bubbles are retained for the longest timepossible and therefore have the greatest opportunity to transfer oxygeninto solution.

In a fully operating system and in accordance with the present inventionanother useful by-product is waste biomass material which can bepackaged and sold e.g. as a fertiliser or fish-feed.

Typical apparatus in accordance with the present invention may include agenerally vertical tower or reactor incorporating means for establishinga bed of material to be fluidised wherein the fluidizing medium isintroduced at the base of the tower to pass upwardly through the mediumto be fluidised.

In one embodiment of this aspect of the invention, the fluidizing mediummay be injected directly to the distributor layer without first passingthrough a perforated or like support plate. In a preferred embodiment,the distributor layer should preferably have a density greater than thatof the fluidised layer and should form a distinct layer below that ofthe fluidised bed.

It will be apparent to the person skilled in the art that in the absenceof a support plate, the particle size of the distributor layer requiredfor effective operation of the fluidised bed is dependent to thevelocity of the fluidizing medium. It is preferred that the particlesize of the distributor layer should be selected such that under theprevailing conditions, the particles constituting the distributor layermove but are not themselves fluidised.

In a further aspect of the present invention the means for establishinga bed of material to be fluidised is preferably a vertical tower orreactor vessel and the fluidizing medium is preferably injected at thebase of the tower to pass upwardly through the medium to be fluidised.

The tower or reactor vessel may be provided with a central conduit forthe supply of fluidizing medium through which the supply of fluidizingmedium passes downwardly toward the base of the reactor or tower wherebythe fluidizing medium is injected downwardly against a reflector elementfor redirection upwardly through the distributor layer and the fluidisedbed. The injection means may include a supply conduit, therefore, whichis sized to have sufficient frictional losses to allow a degree ofcontrol over the flow through it by varying the hydrostatic head.

In the embodiment in which the fluidised bed carries a reactant layer onthe surface of the particulate material constituting the fluidised beditself, the thickness of the reactant layer on the carrier medium may becontrolled by allowing particles containing excess of reactant on thesurface to be removed from the upper part of the fluidised bed and to berecycled and injected into the bed with the fluidizing medium in themanner described above. In an alternative embodiment, substantialbiofilm control can be achieved simply through interactions between themoving bed distributor particles and the biofilm-coated particles.

It will be apparent from the foregoing that there will need to becontrol means for controlling the rate of fluid flow through thereactant bed. Such control means should include means for sampling theoxygen concentration in the fluid before or during entry into thereactor and means for sampling the oxygen concentration of the fluid orliquid on exit or after exiting the reactor. Means may be provided foradjusting the flow rate of fluid through the reactor and/or for sensingother reactor parameters such that the oxygen concentration on leavingthe reactor is just above a concentration at which the oxygenconcentration would be rate controlling for the nitrification process.In one embodiment of the invention, this is about 0.1 to 0.3 mg/l. Theapparatus in accordance with the present invention may also includemeans for aerating the liquid exiting from the reactor vessel. In thisconnection, the aeration may be effected by cascading the liquid overthe top of the reactor and allow it to fall through air for collection.In an alternative embodiment, aeration may be effected by recyclingfluid from the end of the fluidised bed distal to the distributor to theupper end of an aeration column, where oxygen-containing gas bubbles areinjected at or near the lower end and rise, counter-current, to thedescending liquid; thereby transferring oxygen with increasedefficiency. A typical aeration efficiency in a prior art co-currentprocess is in the order of 3-6%; whereas in the present invention,efficiencies of the order of 7-12% in the counter-current process of thepresent invention.

Means may be the provided to separate sloughed biomass from the reactor;such means may be a sedimentation tank or hydrocyclone. Another means ofcontrolling the fluid flow through the system may be effected byproviding header tank means, pump means for pumping fluidizing medium tothe head tank and supply means from said header tank to the injectionmeans for the fluidised bed, the arrangement being such that the headertank provide sufficient hydrostatic pressure at the injection means tomaintain the distributor layer and to effect fluidization of the bed. Itfollows from this, therefore, that control of the flow rate through theapparatus in accordance with the invention may be effected bycontrolling the hydrostatic head in the header tank. In a particularaspect, the injection means may include a supply conduit therefore,which is sized to have sufficient frictional losses to allow a degree ofcontrol over the flow simply by varying the applied hydrostatic head tothe fluid entering the conduit.

Where the method and apparatus of the present invention is used in thepurification of water, it is frequently the case that wastewater isdischarged to a waste tank in which further purification takes place byvirtue of membrane filtration. Such a process is relatively slow andquite expensive to operate. From time to time the membrane “blinds” as aresult of being clogged by suspended matter. We have found surprisinglythat the proportion of suspended matter and biological residues,including suspended, viable bacteria in water purified in accordancewith the present invention is reduced quite significantly with a resultthat membrane filtration treatment subsequent to the nitrificationtreatment in accordance with this invention can proceed much moreefficiently. The average reduction in suspended solids concentrationusing the British Standard method (BS EN 872: 1996 BS 6068: Section2.54: 1996) was 2.4 mg/l, which equated to 21 percent; and the averagereduction in numbers of viable Escherichia coli was nearly 80 percent.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Following is a description by way of example only and with reference tothe accompanying informal drawings of methods of carrying the inventioninto effect.

In the drawings:

FIG. 1 is a graph showing the change in static bed height before andafter the addition of a moving, silica sand bed to assist flowdistribution.

FIG. 2 is a graph showing the rate of abrasion of a fluidised bed ofglassy coke, with and without a moving, silica sand distributor.

FIGS. 3 to 9 illustrate various embodiments of the use of a fixedhydrostatic head for the apparatus in accordance with the presentinvention.

FIG. 10 is a diagrammatic representation illustrating the technique ofbiomass thickness control in accordance with the present invention.

FIG. 11 is a diagrammatic representation of an alternative pumpedversion of the embodiment shown in FIG. 10.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings in which some but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Despite many years experience of using glassy coke as a biomass supportmaterial in laboratory-scale fluidised bed bioreactors, severeturbulence was noted in the lower region of a pilot scale nitrificationplant. The particular plant was established which had flow distributionvia downward discharge into a 60° cone, a common design for this scaleof operation. Although the scientific literature describes a turbulentregion above some types of distributor before the flow is calmed andsmooth fluidization is established, it was thought that this was aproblem peculiar to dense materials such as sand, where higher flowvelocities are required to achieve fluidization. A similar observationwas to some extent made with glassy coke, because the turbulent regiononly extended 30-40 cm up the fluidised bed compared to a 60-100 cmspouted bed region reported for a sand fluidised bed in a similar pilotscale reactor having a porous plate fitted above the inlet cone, see P.F. Cooper, and D. H. V. Wheeldon, “Complete treatment of sewage in atwo-fluidised bed system”, Chapter 7 in P. F. Cooper and B. Atkinson(Editors), “Biological Fluidised Bed Treatment of Water and Wastewater”,Ellis Horwood, Chichester, 1981.

Continued operation of the pilot reactor with coke was characterised bythe production of fines by severe attrition. This led to therecirculating water becoming black and opaque overnight, requiringregular flushing of the reactor to restore clarity, even though the flowrate through the system was equivalent to the entire volume beingreplaced approximately every two hours. Operation was continued, as itwas expected that the eventual formation of a biofilm on the cokeparticles would protect it from further abrasion, because this hadoccurred with the laboratory-scale bioreactors. Unfortunately, suchprotective colonization did not occur. Regular measurements demonstratedthat approximately 0.3% of the original static bed height was lost perday (FIG. 1), meaning that all the coke would wear away in about 10months. Obviously, the rate of attrition exceeded the rate ofcolonization. Previously, upward discharge into an expansion zone hadbeen used for laboratory scale bioreactors, with little evidence of cokeattrition. The high rate of attrition at pilot-scale was all the moresurprising since it had previously been found that glassy coke was muchmore durable than activated carbon.

In an experiment to try to reduce the turbulence-generating effect ofthe inlet flow on the expanded coke bed, a laboratory-scale rig wasused. This rig was fed by upward discharge of tap water via an expansionsection into a bed of glassy coke. At high flow rate, this mimicked theturbulence-generating effect seen with the pilot reactor. Increasingquantities of silica sand were added until there was sufficient (8.5 cm)depth to absorb the force of the inlet flow and distribute it in asufficiently even manner to produce a smoothly fluidised bed of coke,with no sign of turbulence or spouting.

In the pilot scale operation, silica sand was added incrementally to thebioreactor until the turbulence-calming effect was noted. This requireda sand bed 10 cm deep. Once smooth fluidization of the coke had beenachieved, the reactor was flushed to remove the accumulated fines and torestore optical clarity. Even after overnight operation, therecirculating water was still clear: the generation of fines had ceased.In fact, the water remained clear from the time that sand was added as amoving bed flow-distributor: fines generation by attrition of the cokehad indeed stopped. This fact is clear from the static bed height datapresented in FIG. 1. Furthermore, colonization of the coke by nitrifyingbacteria now began, and proceeded at a rate equivalent to almost 0.1% ofthe new static bed height per day. The bed now grew in size, compared tothe steady decrease prior to the addition of sand.

The unexpected effect of adding a layer of silica sand to the base of aglassy coke fluidised bed led to an experiment to compare the degree ofcoke abrasion with and without sand. Two 5 cm diameter columns 50 cmtall were assembled and charged either with a 15 cm depth of glassy cokeor a 5 cm depth of silica sand and a 15 cm depth of glassy coke. Thebeds were fed from a header tank at an upward velocity of 1.5-2.0 cms⁻¹and allowed to overflow into a sedimentation tank. Fine particles ofcoke settled in the tank, were collected by filtration on a daily basisand dried to constant weight. Although fines were still abraded from thecoke bed with the moving sand bed distributor, it was at approximatelyhalf the rate when no sand was present (see FIG. 2). This is surprising,given that the denser sand particles might be expected to collide moreforcefully with the coke than would coke particles on their own.However, because of the greater density of silica sand, it cannot expandand become fluidised at the upward velocities used with glassy cokeparticles of a size suitable for colonization by microorganisms. Indeed,it was surprising to find that the mass of sand per unit area needs tobe such that it exerts a greater “pressure” than the calculated pressuredrop across the sand bed required to achieve expansion prior tofluidization. The sand layer is in a minimally expanded state i.e. notfluidised and this allows the sand bed to distribute the flow but notbecome mixed with the coke. In this way, the rate of coke attritionbecomes less than the rate of microbial colonization, and biofilmformation can begin.

From this experimental evidence, it is apparent that the fluidised layershould be significantly less dense than the moving bed distributormaterial, relying as it does on the expansion and fluidization of theless dense upper layer but not the distributor material. Because silicasand is a commonly used biomass support material for fluidised bedreactors, a more dense material, such as garnet or ilmenite, must beused in combination as a moving bed distributor for such a support.However, this combination would require more energy for bed expansionand fluidization than does the use of glassy coke and silica sand.Alternatively, the use of a moving bed of e.g. silica sand as the flowdistributor will allow the use of biomass or biocatalyst supportmaterials such as activated carbon, which otherwise wears away tooeasily.

Conventionally, wastewater or other aqueous liquid is pumped into thedistributor zone 21 of a fluidised bed in order to induce bed expansion.In wastewater treatment, this accounts for approximately 40% of theenergy required to operate the process. On sloping sites, a gravity-fedsystem is possible, which would considerably reduce the energy cost.Even without a sloping site, using large, efficient pumps to raise thewastewater to header tanks, and thereby provide a hydrostatic head forexpansion of many beds at a time, would be more energy efficient thandelivery by individual pumps. This would save the operators, and hencethe public, money; as well as reduce the environmental damage caused byexcessive energy generation. Moreover, by judicious design it ispossible to regulate automatically bed expansion, without human ormachine intervention, to take account of variations in flow.

A gravity-fed “fluidised bed” is described by R. Badot, T. Coulom, N. deLongeaux, M. Badard and J. Sibony (“A fluidised-bed reactor: the Bioliftprocess”, Water Science and Technology Vol. 29 10-11: 329-338, 1994),but the system as described is not a true fluidised bed. Rather, it is acirculating bed reactor. Furthermore, it is a three-phase system(gas-liquid-solid) rather than a two phase one (liquid-solid). Moreover,it is designed with an upward flow inlet via a cone-shaped expansionzone at the base.

An interesting feature of the present invention involves using a pipe(10) extending downwardly through the bed (12) from an integral orindependently supported overhead tank (FIG. 10). This pipe is sized soas to have sufficient frictional losses to allow a degree of controlover flow through it by varying the hydrostatic head, from virtuallyzero to tens of centimeters, with excess flow into the header tank beingreturned via an overflow device (14). This automatic control system isto allow for a substantial variation in bed expansion, which in turn isto accommodate a substantial change in effluent flow-rate. Such changecan be caused, for example, by diurnal or wet-to-dry weatherfluctuations in effluent flow rate. It will be appreciated that in thetreatment of waste water, in dry conditions the water will have a muchhigher concentration of nitrogen than when it is highly diluted withstorm water.

Pumping costs for water are a significant expense for any wastewatertreatment system. In this aspect of the invention, bed expansion isinduced by a commonly fed, gravity flow system. Because the density ofglassy coke is low, compared to more conventional biomass supportmaterials like silica sand, only a relatively small hydrostatic head isrequired for expansion. For laboratory-scale systems, a 5-30 cmhydrostatic head was adequate to produce sufficient flow for good bedexpansion. Scale-up (to e.g. 50-200 cm diameter, 1-5 m tall columns)does not entail a significant scale-up of the hydrostatic head, otherthan to take into account any additional frictional losses.

Calculations from laboratory-scale operation, based on the use of eitherartificial feed or activated sludge effluent, demonstrate that almostcomplete nitrification can be achieved either by sequential passagethrough a series of beds, where the number of beds is equal to the inletammonia concentration minus the required discharge concentration ÷2 orby recycling several times with a cascade of only several beds where thenumber of recycles and beds is calculated in a similar manner. Thesecalculations are based on an unpressurised system where oxygen issupplied from air. With pressurized systems or systems where oxygenenrichment is used, the number of beds or recycles can be reducedpro-rata to the increase in dissolved oxygen concentration therebyachieved. During high flow conditions typical of wet weather, when theammonia is more dilute (e.g. 6 mg NH₃—N dm⁻³), a single pass througheach of only two or three columns will be sufficient (see FIG. 3).Counterintuitively, it is calculated that pumped flow for recycle willonly be required for lower, dry weather flows, because then the ammoniais more concentrated (up to 25 mg NH₃—N dm −3) Thus, our novel designonly requires pumping of the minimal volume of wastewater. Even for dryweather flows, only a single recycle will be required, therebyminimising the energy requirements. Thus, pump size, cost, and energyconsumption will be minimised.

At other than sloping sites, the wastewater will need to be pumped to aheight above the top of the highest bed to produce sufficienthydrostatic head for bed expansion. From there it will flow by gravity,except for recycle flow under conditions of dry weather or high ammoniaconcentration. By judicious design, common pumping stations and headertanks will minimise construction and operating costs. However, thisdesign efficiency is dependent on having a manifold made to produceequal flow at each fluidised bed module. FIGS. 3-7 illustrate variousconfigurations of fluidised bed modules in cascade arrangements.Obviously, there is no need to place one module above another ifeffluent is pumped to each module, e.g. at sites where sufficient landarea is available and pumping costs are not prohibitive (FIGS. 8 and 9).

In the activated sludge process for secondary (biological) treatment ofsewage, the supply of compressed air accounts for up to 60% of energycost. Where the oxygen demand of the biological process is high,oxygen-enrichment of the supplied air is required, which is even moreexpensive.

In conventional biological fluidised bed processes for wastewatertreatment, such as nitrification, oxygen is supplied to themicroorganisms either by bubbling air through the bed, see for exampleU.S. Pat. No. 4,490,258 or by supplying pre-aerated effluent to the bed,see UK Patent No 1520895. The first solution suffers from excessivebiomass stripping from the support particles; whilst the latter solutionis expensive. Air is expensive to compress and, when supplemented withoxygen, aeration is even more expensive. In some systems, the air isentirely replaced with oxygen, occasioning expensive and hazardousstorage facilities.

Expense notwithstanding, systems to supply dissolved oxygen are designedto be operated under pressure, necessitating expensive pressure vesselsof difficult to fabricate shape. Operation under pressure increases theoxygen carrying capacity of the wastewater, according to Henry's Law.However, increased dissolved oxygen concentration can lead to at leasttwo problems when discharged to the fluidised bed. First, the lowerpressure in the bed causes de-gassing, with the resultant bubblescausing biofilm to be stripped from the support material particles.Second, the higher dissolved oxygen content causes oxidative stress tothe bacteria, leading to the diversion of energy and materials from thedesired biological process and into repair and protection of the cells.Third, the release of gas bubbles into a fluidised bed tends to convertit into a re-circulating bed, resulting in the establishment of rapidvertical mixing, thereby disrupting the formation of physical, chemicaland biological gradients.

Another consequence of an air circulating bed is to cause the oxygenconcentration to be equilibrated throughout the volume of the liquid inthe vessel. Although most aerobic biological processes require only lowlevels of dissolved oxygen, they tend to be controlled at a minimumconcentration of 2 mg dm⁻³, which approximates to 20-30% saturation withrespect to air. Not only does this decrease the driving force for oxygentransfer but it also means that zones of different oxygen concentrationcannot be achieved. In natural biological systems, gradients of nutrientconcentration, including oxygen, are important for establishingdifferent populations of microbes, each suited to different biologicalprocesses. The relationship between the dissolved oxygen concentrationat the top of the bed and the residual ammonia concentration in thetreated effluent indicates the dissolved oxygen concentration controlpoints for a range of residual ammonia concentrations. For example, adissolved oxygen concentration greater than 0.3 mg/l is required toachieve a residual ammonia concentration less than 1 mg/l

Operation of a fluidised bed of glassy coke colonised by a nitrifyingbiofilm can lead to complete depletion of the dissolved oxygen as thewastewater passes up through the bed, giving high rates ofnitrification, despite industry guidelines for maintaining dissolvedoxygen at a minimum concentration of 2 mg dm⁻³. Operation without gasbubbling in this way allows the dissolved oxygen concentration to falllow enough to allow de-nitrifying bacteria to use the nitrite and/ornitrate produced by the aerobic nitrifyers lower down. It is, therefore,possible to have a denitrification zone above the nitrification one, allin the same bioreactor. This significantly improves the spaceutilisation and operating cost efficiency of the system. Furthermore, onexit from the bed, the virtually oxygen-free wastewater rapidly absorbsoxygen from air or other oxygen-containing gas, flowing or bubblingcounter-current to the wastewater recycle flow, causing the dissolvedoxygen to be raised to a concentration in excess of 85% in a matter ofseconds. Moreover, the efficiency of this counter-current oxygentransfer is more than double that of the more conventional co-currentaeration (7-12% oxygen removed compared to 3-6%).

It is well known that the driving force for oxygenation is proportionalto the difference in partial pressure between the gas phase (air, 100%)and the liquid phase (water, initially at or near 0%). Furthermore,allowing oxygen-depleted water to cascade for a distance of as little as100 cm down the outside of a column also causes rapid re-aeration,reaching a value of 45-80% at the bottom.

Allowing the wastewater to overflow the top of the expanded bed columnand run down its sides creates a thin film, which helps to maximise therate of oxygen transfer. To achieve fluidisation of small glassy cokeparticles, the wastewater rises up the bed at between 0.5 to 2.0 cm/sec,thereby taking between 50-200 seconds to rise 1.0 m. In contrast, asheet of water in contact with a vertical surface falls at almost 1.5m/sec (Grassmann, P. Physical Principles of Chemical Engineering,Pergamon Press, 1971). Thus, a film of between 40-160 μm can be expectedto form around a 10 cm diameter column, and one of 400-1600 μm for a 50cm diameter one. The high slip velocity between the gas and liquidphases minimizes the thickness of the laminar boundary layer, therebymaximising the rate of oxygen transfer. It has been established for a 10cm diameter column that a high degree of oxygenation (45-80% dissolvedoxygen concentration) can be achieved with a fall of just 1.0 m. Similarresults are expected for a 50 cm column; especially if the fall, degreeof turbulence or surface area of water film is increased, therebycausing an increase in the oxygenation rate.

It follows therefore that by building fluidised bed modules with aheader tank, for feeding wastewater and causing bed expansion,re-aeration of the wastewater can be optimised by allowing it to flowdown the outside of a reactor vessel (FIGS. 4-8). In this way, the majorenergy costs of supplying oxygen are largely dispensed with.

Whilst the encouragement of microbial growth as attached biofilm onsmall particles of biomass support material for operation as a fluidisedbed gives clear process advantages, it does create the problem ofbiofilm overgrowth. Investigations of the kinetics of biochemicalconversions in microbial biofilm indicate that as the film thicknessincreases, cells further than about 0.1 to 0.15 mm (100-150 μm) from theouter surface become starved, particularly of oxygen (M. Denac, S.Uzman, H. Tanaka & I. J. Dunn, “Modelling and experiments on biofilmpenetration effects in a fluidised bed nitrification reactor”,Biotechnology and Bioengineering Vol. 25: 1841-1861). Control of biofilmthickness can, therefore, have significant advantages in terms ofprocess efficiency, by ensuring that the majority of cells in thebiofilm are supplied with sufficient nutrients or oxygen.

Jeris, Beer and Mueller of Ecolotrol in U.S. Pat. No. 3,956,129 describeseveral methods for biofilm control by removal and mechanical agitation.These methods included a mixer with “a rotating blade similar to aWaring Blender”, or the “use of compressed air or water sprays”. Inlater inventions, Jeris discloses “a rotating flexible stirrer” at thetop of the bed U.S. Pat. No. 4,009,098, a mechanical stirrer mounted inthe top of the bed U.S. Pat. No. 4,009,105 and U.S. Pat. No. 4,009,099)or “rotating a sharp blade or flexible agitator” at the top of the bedGB 1520895 or “rotating a sharp blade or flexible stirrer” at the top ofthe bed U.S. Pat. No. 4,009,099. Later work at Ecolotrol, by Hickey andOwens, disclosed a control system based on a separator column within theupper portion of the fluidised bed, which relied on a variety ofagitator arrangements for biofilm stripping see EP Patent 0007783. Theseagitator arrangements included a motor-driven blade, a transducer toproduce sonic energy, a pump for removal of particles with thick biofilmand return of particles stripped of biofilm, and a similar pumped systembut with a static line mixer or a means to effect hydraulic shearing.One of the more novel approaches to biofilm control has been describedin U.S. Pat. No. 4,618,418, where support particles coated with thickbiofilm are carried by gas lift to a rim where they overflow into asettling zone. These particles are then carried down to a point “ . . .preferably somewhat below halfway the distance between the roof of thereaction space and the liquid distribution device . . . ”, for re-entryinto the fluidised bed.

In all the above cases of biofilm removal devices and methods,significant energy and mechanical equipment are required to achievecontrol, the latter also requiring periodic maintenance and replacement.Moreover, the stripped biofilm must be wasted from the system andde-watered prior to disposal, with the stripped biomass support materialparticles recovered for return to the bed.

In the present invention biofilm control can be performed with minimalenergy cost and in a system with no moving parts. In this way, costs forinstallation, operation, maintenance, and replacement are minimised.Normally, the bed will expand through biofilm growth, with the particlescarrying thicker biofilm tending to migrate towards the top of the beddue to their decrease in density. If an overflow device (14) isincorporated at a point in the bioreactor (8) where bed height is to becontrolled, then further growth of the biofilm will cause the mostthickly-coated particles to enter the overflow device (14) (FIG. 10).These particles are caused to flow, under the combined influence ofgravity and flow induced by a venturi injector (16) positioned justbefore the end of the down flowing inlet stream (18) (FIG. 10). Thefluidising medium then impacts on the lower wall (20) of the reactorvessel and reverses its flow upwardly and through the distributor layer(21) and the fluidised bed (12).

Alternatively, with pumped systems, the overflow device and venturi canbe external, but again positioned just before the inlet to the bed (FIG.11). In this way, particles with thick biofilm re-enter the bed in themost turbulent region, that of the distributor. Furthermore, if thatregion contains a bed of small, dense, mobile particles (e.g. silicasand) below a less dense bed of biomass support material (e.g. glassycoke) then even more efficient biofilm stripping can occur as thebiofilm-coated coke passes through the lower, moving bed and the lowerregion of the upper, fluidised bed.

A number of comparative experiments were conducted to test theeffectiveness of the distributor layer in assisting in biofilmstripping. Plots were made one for the expanded bed and moving beddistributor in accordance with the present invention and the otherwithout the distributor material.

Further advantages of this approach include the automatic return ofbiomass support material to the bed. Although these particles will havebeen stripped of their thick biofilm, they will still retain asufficient number and mixture of bacteria to allow the rapidre-development of fresh biofilm. In this way, moribund cells are removedand replaced by fresh ones. Not only that, but the stripped cells areretained in the system, where active ones can help re-colonize strippedparticles and also contribute to the overall performance of the systembefore eventually being washed out. Biomass leaving the system willeither be as biofilm particles or caused to aggregate into biofilm flocsbecause of the hydrodynamic conditions during passage through the bed.This aggregated microbial matter is significantly more easily separatedfrom the wastewater, either by sedimentation, centrifugation,filtration, or other well-known techniques. In particular, allowing thetreated wastewater to exit the system via a hydrocyclone willconcentrate the biomais in the most energy efficient manner.

In an alternative embodiment, recycle of thicker biofilm-coatedparticles from the top of the bed via an injector is not required. Withthe correct balance between inlet fluid velocity and moving beddistributor particle momentum, there is sufficient interaction with theoverlying fluidised bed of bioparticles to substantially effect biofilmcontrol.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method of treating a liquid by passing said liquid through afluidised bed of glassy coke carrier particles in which a biomass isimmobilized as a biofilm grown on the surface of the carrier particlesconstituting the fluidised bed, whereby said bed is fluidised by thepassage of said liquid therethrough, and wherein the liquid is caused orallowed to pass through a particulate distributor layer prior to passagethrough the fluidised bed, the carrier particles and the distributorlayer being contained together in a single enclosure and there being nobarrier between the carrier particles and the distributor layer; whereinthe density of the particles of the distributor layer, and the flow rateof the liquid are selected such that the distributor layer is agitatedby the liquid but not fluidised, and turbulence in the liquid is reducedor eliminated before acting on the fluidised bed, and wherein thedensity of the particles of the distributor layer is greater than thatof the carrier particles.
 2. A method as claimed in claim 1, wherein thedistributor layer is a distinct layer below the fluidised bed and has aminimal overlap with the fluidised bed to reduce abrasion and/or removalof the reactant from the carrier particles.
 3. A method as claimed inclaim 1, wherein the thickness of the distributor layer and the liquidflow rate are selected such that substantially no turbulence isexperienced in the fluidised bed as a result of passage of thefluidising medium therethrough.
 4. A method as claimed in claim 1,wherein the glassy coke particles have a “glassy” or slightly glazedsurface and a size within the range of 0.25 to 2.50 mm.
 5. A method asclaimed in claimed 4, wherein the glassy coke has a particle size of 0.7to 1.0 mm.
 6. A method as claimed in claim 1, wherein the liquid is amedium to be nitrified and the biofilm comprises a nitrifying bacteria.7. A method as claimed in claim 6, wherein the temperature of the liquidis controlled to within the range of 13 to 22 degrees centigrade.
 8. Amethod as claimed in claim 6, wherein the fluidised bed contains anupper denitrification zone above a nitrification zone, whichnitrification zone comprises aerobic nitrifying bacteria, said upperzone incorporating a denitrifying bacteria to break down the nitriteand/or nitrate produced by nitrifying bacteria in the nitrificationzone.
 9. A sewage treatment process comprising a nitrification method asclaimed in claim
 6. 10. A water purification process comprising anitrification method as claimed in claim
 6. 11. A method as claimed inclaim 1, wherein the liquid is exhausted from a top of the bed, andcausing or allowing the liquid to overflow or cascade as a thin filmthrough air to effect aeration of the liquid.
 12. A method as claimed inclaim 1, wherein said liquid is subjected to further purification bymembrane filtration, after passage through the fluidised bed.
 13. Amethod as claimed in claim 1, further comprising exhausting the liquidfrom a top of the bed, and recycling it through a counter-currentaerator.
 14. A method as claimed in claim 13, further comprisingbubbling air, oxygen-enriched air or pure oxygen upwards through acolumn of said counter-current aerator, and passing said liquiddownwardly through said column, which column is sized such that thedownward velocity of said liquid is slightly less than the natural risevelocity of the gas bubbles, whereby the gas bubbles are retained forthe longest time for transferring oxygen into solution.
 15. A method oftreating a liquid by passing said liquid through a fluidised bed ofglassy coke carrier particles in which a biomass is immobilized as abiofilm grown on the surface of the carrier particles constituting thefluidised bed, whereby said bed is fluidised by the passage of saidliquid therethrough, and wherein the liquid is caused or allowed to passthrough a particulate distributor layer prior to passage through thefluidised bed; wherein the density of the particles of the distributorlayer, and the flow rate of the liquid are selected such that thedistributor layer is agitated by the liquid but not fluidised, andturbulence in the liquid is reduced or eliminated before acting on thefluidised bed, and wherein the thickness of the biofilm is controlled byremoving carrier particles of the fluidised bed containing excessbiofilm on the surface from an upper part of the fluidised bed andrecycling them into the distributor layer, such that excess biofilm isstripped from the carrier particles by the action of the distributorlayer as the carrier particles pass therethrough towards the fluidisedbed.
 16. A method as claimed in claim 15, wherein the recycled carrierparticles are combined with the liquid for introduction to thedistributor layer.
 17. Apparatus for treating a liquid comprising: a bedof particulate material to be fluidised, said bed being constituted byglassy coke carrier particles having immobilized thereon a biofilm as areactant moiety; a particulate distributor layer made up of aparticulate material having a density greater than that of the carrierparticles; a single enclosure containing both the carrier particles andthe distributor layer, there being no barrier between the carrierparticles and the distributor layer; an injector for injecting a streamof liquid to be treated through said bed via the distributor layer; anda control device for controlling the flow rate of said liquid throughthe fluidised bed so as to fluidise the carrier particles, the densityof the particles of the distributor layer and the flow rate of theliquid being selected such that the distributor layer is agitated by theliquid, but not fluidised, and turbulence in the liquid as it passesthrough said fluidised bed is substantially reduced.
 18. Apparatus asclaimed in claim 17, wherein said apparatus is configured for allowingsufficient interaction between the distributor layer and the biofilmcarried by said carrier particles to effect biofilm thickness control.19. Apparatus as claimed in claim 17, wherein the distributor layer hasa density greater than that of the fluidised bed.
 20. Apparatus asclaimed in claim 17, wherein the distributor layer is a distinct layerbelow that of the fluidised bed.
 21. Apparatus as claimed in claim 17,wherein the bed of particulate material to be fluidised is disposed in avertical tower or reactor having a base, and said injector device isarranged to inject the liquid at the base to pass upwardly through thefluidised bed.
 22. Apparatus as claimed in claim 21, wherein the toweror reactor is provided with a central conduit for supplying said liquiddownwardly to the base of the reactor or tower, and a reflector elementat said base, whereby the liquid is injected against said reflectorelement for redirection upwardly through the distributor layer and thefluidised bed.
 23. Apparatus as claimed in claim 17, wherein the carrierparticles carry a layer of a biomass comprising nitrifying bacteria, andhave a size within the range 0.25 to 2.50 mm, and carry a biofilm ofsaid nitrifying bacteria on the surface thereof.
 24. Apparatus asclaimed in claim 23, wherein said control device comprises a device forsampling the oxygen concentration in the liquid before or during entryinto the fluidised bed, a device for sampling the oxygen concentrationof the liquid at or after the exit of the fluidised bed, and a devicefor adjusting the flow rate of liquid through the fluidised bed, and/orother parameters, such that the oxygen concentration on leaving thefluidised bed is just above a concentration at which the oxygenconcentration would be rate controlling for the nitrification process.25. Apparatus as claimed in claim 17, wherein said apparatus isconfigured for aerating liquid exhausted from a top of the fluidisedbed.
 26. Apparatus as claimed in claim 25, wherein said apparatuscomprises a counter-current aerator.
 27. Apparatus as claimed in claim26, wherein said counter-current aerator comprises a column, a devicefor directing said exhausted liquid downwards through said column, and adevice for bubbling air, oxygen-enriched air or pure oxygen upwardsthrough the column, wherein the column is sized such that the downwardvelocity of said liquid is slightly less than the natural rise velocityof the gas bubbles, whereby the gas bubbles are retained for the longesttime for transferring oxygen into solution.
 28. Apparatus as claimed inclaim 25, wherein said apparatus is configured for causing or allowingliquid exhausted from a top of the fluidised bed to overflow or cascadeas a thin film to effect rapid aeration thereof.
 29. Apparatus asclaimed in claim 17, further including a device to separate sloughedbiomass from the fluidised bed.
 30. Apparatus as claimed in claim 17,further including a header tank, a pump for pumping said liquid to saidheader tank, and a supply device from said header tank to the injectiondevice, whereby the header tank provides sufficient hydrostatic pressureat the injection device to maintain the distributor layer and to effectfluidisation of the bed.
 31. Apparatus as claimed in claim 30, whereinthe injection device includes a supply conduit therefor, which is sizedto have sufficient frictional losses to allow a degree of control overthe flow through it by varying the hydrostatic head.
 32. Apparatus asclaimed in claim 17, wherein the distributor layer is silica sand. 33.Apparatus as claimed in claim 17, further comprising a membranefiltration device for further purifying the liquid after passage throughthe fluidised bed.
 34. Apparatus for treating a liquid comprising: a bedof particulate material to be fluidised, said bed being constituted byglassy coke carrier particles having immobilized thereon a biofilm as areactant moiety; a particulate distributor layer; an injector forinjecting a stream of liquid to be treated through said bed via thedistributor layer; and a control device for controlling the flow rate ofsaid liquid through the fluidised bed so as to fluidise the carrierparticles, the density of the particles of the distributor layer and theflow rate of the liquid being selected such that the distributor layeris agitated by the liquid, but not fluidised, and turbulence in theliquid as it passes through said fluidised bed is substantially reduced;wherein said apparatus is configured for removing carrier particlescontaining an excess of biofilm from an upper part of said fluidisedbed, and a secondary injection device is provided juxtaposed thedistributor layer, said secondary injection device being arranged tointroduce said carrier particles removed from the upper part of saidfluidised bed into the fluidised bed via said distributor layer, wherebyexcess biofilm is stripped from the carrier particles by the action ofthe particles of the distributor layer as the carrier particles passupwardly towards the fluidised bed.